1. Field of the Invention
This invention relates to a novel monostable ferroelectric liquid crystal display apparatus employing a liquid crystal material having a chyral smectic C-phase.
2. Description of the Related Art
Up to now, a liquid crystal display device is extensively used as a lightweight thin display device. In particular, a TFT driven twisted nematic (TN) display device is preferentially used for this purpose.
However, the above display device suffers the problem of gradation reversing behavior in the half tone, a narrow angle of field and slow response time amounting to tens of milliseconds (msec). In particular, the response time between the half tones exceeds 100 msec, such that there is observed the phenomenon of bleeding due to delayed picture display.
Under this situation, a ferroelectric liquid crystal is stirring up notice as a liquid crystal material which not only improves the angle of field of view and simultaneously shortens the response time, and the possibility of application thereof to display is now under investigation.
It has been proposed by Clark et al in Appl. Phys. Lett. 36,899 (1980), U.S. Pat. No. 4,367,924 and Japanese Patent Publication S-60-22287 a passive matrix driven type surface-stabilized ferroelectric liquid crystal display SSFLCD exploiting the bistability having two memory states obtained on unravelling the pitch of the helix of the ferroelectric liquid crystal in a narrow gap cell.
This SSFLCD has an advantage that it can realize a monochromatic display device of a wide angle of field
However, although this SSFLCD has a response time per pixel as short as tens of microseconds (xcexcsec), imposed by the passive matrix driving method, a high speed is not necessarily realized in a display having a large number of pixels. In particular, the SSFLCD is not applicable to moving picture display suited to the demand of the multi-media era.
Moreover, the number of pixels is also gradually increasing from so-called VGA through SVGA, XGA, SXGA and UXGA, in this sequence, such that display of a large capacity may now be realized. Thus, the principle of high speed display in the true sense of the term has been a desideratum.
For realizing this high speed response and wide field of angle display, the present Assignee has proposed a monostable FLC mode in Japanese Laying-Open Patent H-4-212126 or in U.S. Pat. No. 5214523.
This mode uses a liquid crystal device comprised of a pair of substrates processed with uniaxial orientation. These substrates are arranged facing each other with the respective directions of orientation processing parallel to each other, and a liquid crystal material having a chiral smectic C phase charged between the two substrates. The component of projection to the substrate of an axial direction of a cone delineated by the liquid crystal molecules of the liquid crystal material having this chiral smectic C phase and the component of projection to the substrate of the direction of the molecular axis of the liquid crystal molecule itself are set so as to be coincident with the direction of the orientation processing of the substrate. The mode has the monostable structure with the aforementioned state as an initial state. A switching element is provided for each pixel as the smallest unit in order to realize the active matrix driving. The director of the liquid crystal (tilt of the molecular axis) is changed continuously by application of an electrical voltage and the intensity of the transmitted light is modified in an analogous fashion to realize analog gradation representation with a wide angle of field and full color representation.
Specifically, the transparent electrode side of a glass plate having a transparent electrode (ITO) is processed with a silane coupling and coated with a film of a polyamide acid by a spin coating method. This film is imidized on baking to form a polyimide film. This polyimide film is processed with rubbing in one direction with a velvet cloth to provide an oriented film. The oriented film has a thickness of approximately 20xc3x85 and exhibits asymmetry with respect to the rubbing direction which produces the effect of the liquid crystal orientation. The glass plate, on which the oriented film has been produced as explained above, is arranged facing a transparent electrode so that the rubbing direction will be antiparallel to each other. Then, a cell with a gap of 2 xcexcm is assembled using a UV cured adhesive comprised of micro-pearls 2 xcexcm in diameter. The monostable FLC mode is realized using the following compound (a): 
as a chiral component, and also using a tricyclic difluorine based compound (b), a phenyl pyrimidine based compound (c) and a phenyl benzoate based compound (d), as non-chiral components: 
However, with the above liquid crystal compound, sufficient contrast has not been achieved due to the insufficient black level.
On the other hand, alkyl chains coupled to each other through an ether linkage or the alkyl chain directly coupled to a phenyl pyrimidine skeleton is used as a structure for each terminal of phenyl pyrimidine contained in the conventional liquid crystal composition.
However, if simply a dicyclic or tricyclic phenyl pyrimidine is used as a base liquid crystal as described in e.g., Japanese Laying-Open Patent H-9-165578, a bistable structure results, such that the monostable mode cannot be realized.
There are a variety of alkyl chains of the phenyl pyrimidine based liquid crystal of the phenyl pyrimidine based crystal, such that, if a liquid crystal composition is constituted without limiting the sorts of the alkyl chains, and this liquid crystal composition is injected into a liquid cell, the black level is not improved even on demonstrating the monostable mode. The major portion of the liquid cell is in the bistable state such that stable monostable mode cannot be realized.
The result is that the analog gradation by the active matrix driving proper to the nonostble FLC mode is not sufficiently obtained. On the other hand, the black level sinking is not sufficient and hence the contrast which satisfies the high quality display device is not obtained.
It is therefore an object of the present invention to provide a monostable ferroelectric liquid crystal display apparatus in which it is posible to realize stable monostable mode to achieve sufficient analog gradation and contrast.
The present invention provides a monostable ferroelectric liquid crystal display device made up of a pair of substrates, which have been subjected to uniaxial orientation processing, and which are arranged facing each other so that the respective directions of the orientation processing will be subsequently parallel to each other, and a ferroelectric liquid crystal material which has a chiral smectic C phase, and which is charged between these substrates, with a projection component on the substrates of an axial direction of a cone drawn by a liquid crystal molecule of said ferroelectric liquid crystal and with a projection component on the substrates of an axial direction of the liquid crystal molecule itself being subsequently equal to the direction of the orientation processing of the substrates, with the liquid crystal device being mono-stabilized with this state as an initial state, wherein the ferroelectric liquid crystal material is a liquid crystal composition containing a compound comprised of a phenyl pyrimidine skeleton on both terminals of which are linked alkyl chains the sum total of carbon atoms of which is equal to 17.
By the addition of dicyclic phenyl pyrimidine, comprised of a phenyl pyrimidine skeleton, on both terminals of which are linked alkyl chains the sum total of carbon atoms of which is equal to 17, a stable monostable ferroelectric liquid crystal display device mode is realized, although the precise mechanism is not known. Thus, the resulting liquid crystal device is improved in black level and in the contrast ratio.
Therefore, with the present invention, a monostable ferroelectric liquid crystal display device is achieved, in which a stabilized monostable mode is realized to achieve sufficient analog gradation and contrast.